Tuning photovoltaic performance of DOBT-based dyes via molecular design with ethynyl-linker and terminal electron-donating segment

Article abstract of DOI:10.1016/j.dyepig.2017.01.044

Two novel D-π-A-π-D-typed dioctyloxy-benzothiadiazole (DOBT)-based small molecules (SMs), TPAEDOBT and CZEDOBT, were designed and synthesized, consisting of ethynyl as π-linkage, besides, incorporating triphenylamine (TPA) and alkylated-carbazole (Cz) as

Full text of DOI:10.1016/j.dyepig.2017.01.044

Dyes and Pigments 140 (2017) 203e211
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Tuning photovoltaic performance of DOBT-based dyes via molecular
design with ethynyl-linker and terminal electron-donating segment
,
, *
Lihui Wang a b, Lunxiang Yin a, Lijuan Wang a, Bao Xie a, Changyan Ji a, Yanqin Li a
a School of Chemistry, Dalian University of Technology, Linggong Road 2, Dalian, 116024, China
b College of Chemistry and Chemical Engineering, Inner Mongolia University for Nationalities, Tongliao, 028023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel D-
p
-A-
p
-D-typed dioctyloxy-benzothiadiazole (DOBT)-based small molecules (SMs), TPAE-
Received 29 August 2016
Received in revised form
17 January 2017
Accepted 18 January 2017
Available online 19 January 2017
DOBT and CZEDOBT, were designed and synthesized, consisting of ethynyl as
p
-linkage, besides,
incorporating triphenylamine (TPA) and alkylated-carbazole (Cz) as terminal electron-donating units
respectively. Single-bond compounds, TPADOBT and CZDOBT, were designed as references to fully
investigate their photovoltaic (PV) performance of solution-processed devices. As expected, the PV
properties were finely tuned via molecular design with ethynyl-linker and terminal electron-donating
units. Among them, the best power conversion efficiency (PCE) of 3.21% was obtained for TPAEDOBT-
based device due to triple-bond effect and high hole-mobility. Most importantly, a remarkably increased
open-circuit voltage (Voc) of 1.06 V was achieved for CZEDOBT-based device due to synergistic effect of
ethynyl-linker with electron-withdrawing character and Cz unit with the relatively weakened electron-
donating ability, which is among the earliest report for DOBT-Cz based SMs with high Voc so far. These
results provide an important guide for rational design of novel SMs PV materials.
Keywords:
Benzothiadiazole
Ethynyl
Carbazole
Triphenylamine
Photovoltaic performance
Bulk heterojunction
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
also well control the energy levels [7]. Therefore, the design of SMs
containing appropriate D and A units provide a pathway to
Organic photovoltaics (OPV) have been intensively investigated
during the past decade due to their advantages of low production
costs, high mechanical flexibility, light weight, and simplified
fabrication [1] To date, polymer based bulk heterojunction (BHJ)
organic solar cells (OSCs) have reached PCEs in excess of 10% in
single-junction devices [2]. In the meanwhile, solution processed
SMs based OSCs have made a rapid improvement due to the
intrinsic benefits of easier chemical modification, well-defined
molecular structure and good reproduction [3]. Several SMs-OPV
materials with PCEs near 10% have been achieved [4]. In partic-
ular, a remarkable PCE of 10.10% has been reported by Chen group
[5], which is comparable with their polymer counterparts. It is
worth mentioning that these intrinsic characteristics and the rapid
development have made organic SMs become an excellent candi-
date for OPV materials [6].
modulate PV properties of materials [8]. Among a variety of
methods for improving Voc of OPV device, modification of -bridge
p
in the backbone of SMs is an effective approach [9]. For instance, Li
et al. have reported that a typical compound BDCTFBT with
acrylonitrile-linkage exhibited an impressive Voc of 1.11 V [10].
Among many studies, ethynyl-linker based SMs have received great
attention owing to the intrinsic characteristics of materials, which
include improved coplanarity, rigidity and electron-withdrawing
character [11]. Moreover, SMs with ethynyl-linkage have the po-
tential to improve Voc owing to a deep highest occupied molecular
orbital (HOMO) level [12]. However, the BHJ OPV devices with Voc
over 1.0 V were seldom reported by using the ethynyl-based SMs as
a donor [13]. In this paper, TPA and Cz units were selected as ter-
minal groups due to their prominent electron-donating character
and good charge transportation capacity [14]. On the other hand,
DOBT was designed as central acceptor unit due to its excellent
solution compatibility and strong electron-withdrawing ability
[15]. Although great attention has been paid to TPA, Cz and BT by
Developing rational donor-
p-acceptor (D-p-A) conjugated SMs
is the key to high performance, because high quality D-
p
-A archi-
tecture can not only efficiently reduce the energy band gap (Eg), but
reason of their advantages for OSCs, DOBT-ethynyl-TPA based D-
A- -D typed SMs with PCEs over 2% were seldom reported [11a,13].
Furthermore, DOBT-ethynyl-Cz based SMs have never been studied.
p-
p
* Corresponding author.
E-mail address: liyanqin@dlut.edu.cn (Y. Li).
http://dx.doi.org/10.1016/j.dyepig.2017.01.044
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

204
L. Wang et al. / Dyes and Pigments 140 (2017) 203e211
Therefore, it is highly desired to develop the synergistic effect of
ethynyl-linker and electron-donating units (TPA and Cz), which will
be important for tuning the OPV performance via molecular design.
In this work, two new DOBT-based SMs, TPAEDOBT and CZE-
AVANCE II 400-MHz spectrometer using CDCl3 as solvent and TMS
as the internal standard. High-resolution mass spectra (HRMS)
were collected with
a MALDI Micro MX spectrometer. The
UVeVisible absorption spectra of the synthesized materials were
obtained with an Agilent Cary 5000 spectrophotometer. Fluores-
cence emission spectra and fluorescence quenching experiments
were performed on a Shimadzu RF-5301PC spectrofluorometer.
Cyclic voltammetry (CV) measurements were conducted using a
CHI610D electrochemical workstation from CH instrument, Inc., it
was recorded in 0.1 M anhydrous CH2Cl2/Bu4NBF4 solution under
N2 atmosphere, and ferrocene-ferrocenium (Fc/Fcþ) couple was
selected as internal standard. The glass-carbon electrode was
employed as the working electrode, while Ag/Agþ electrode (Ag in
0.1 M AgNO3 solution of MeCN) and platinum wire electrode were
served as the reference and the counter electrode, respectively. The
corresponding energy levels (HOMOCV and LUMOCV) and the
electrochemical band gaps (ECgV) could be obtained from onset
oxidation potentials (Eox) and onset reduction potentials (Ered) ac-
DOBT, were synthesized successfully, consisting of ethynyl as
p-
bridge, besides, incorporating TPA and Cz as terminal electron-
donating units, respectively. In order to fully investigate the syner-
gistic effect of p-linker and donor unit, two single-bond compounds
named TPADOBT and CZDOBT were designed as reference mate-
rials. Molecular design strategy and chemical structures of these
materials are shown in Scheme 1. Independent and synergistic ef-
fects of p-linker and D units were investigated systematically. As a
consequence, the OPV properties were finely tuned via molecular
design with ethynyl-linker and terminal electron-donating units.
2. Experimental section
2.1. Reagent and materials
cording to following empirical equations [16]: LUMOCV
/
ferrocene
HOMOCV ¼ - [Ered/ox - E
þ 4.8] eV and ECgV ¼ LUMOCV
-
1/2
All reagents and chemicals were obtained commercially and
used as received unless specified. Toluene and tetrahydrofuran
(THF) were pretreated by distillation over Na/benzophenone under
nitrogen (N2) atmosphere prior to use. All reactions and manipu-
lations were performed under N2 atmosphere with a standard
Schlenk line unless otherwise noted.
HOMOCV, where Eox and Ered are the measured onset potentials
ferrocene
relative to Ag/Agþ, E
¼ 0.05 eV versus Ag/Agþ. Thermogra-
1/2
vimetric analysis (TGA) was performed under N2 atmosphere with
a heating rate of 10 ꢁC minꢀ1 using a TA Instruments-Q50 ther-
mogravimetric analyzer. The current density-voltage (J-V) curves of
OPV devices were recorded with a Keithley 2400 Source Measure
Unit under the simulated AM 1.5 G illumination with an intensity of
100 mW cmꢀ2, calibrating with the standard silicon solar cell. The
monochromatic incident-photon-to-electron conversion efficiency
(IPCE) spectra were collected using a SM-25 photoelectric conver-
sion analyzer. The hole mobility (mh) measurements were con-
ducted in the dark with a computer-controlled Keithley 2400
Source Measure Unit system. The mh values can be derived by fitting
the J-V curves to a space-charge-limited currents (SCLC) model in a
double logarithmic scale according to the Mott- Gurney law [16]:
J ¼ (9/8) ε0 εr mh (V2/L3), where ε0 is the permittivity of free space, εr
is the relative permittivity and assumed as approximately 3.0, mh is
the hole mobility value, V is the effective voltage, L is the thickness
of the active layer blend, and J is the current density. Atomic force
microscopy (AFM) measurement of the active layer was carried out
using DI Nanoscope Dimension 3100 atomic force microscope.
2.2. Device fabrication
Solution processed OPV devices were fabricated with a con-
ventional configuration of ITO/PEDOT:PSS/SMs-donor:PC61BM/Al.
The ITO-coated glass substrate was cleaned in water, methanol,
acetone, toluene and isopropyl alcohol for 10 min, successively.
Then a thin layer of poly (3,4-ethylenedioxythiophene):poly (sty-
rene sulfonate) (PEDOT:PSS) was spin-coated onto the substrate at
4000 rpm sꢀ1 for 60 s. After being baked at 140 ꢁC for 20 min, the
prepared substrates were transferred into a N2-filled glove box.
Subsequently, the active layer with a blend solution of SMs-donor
and PC61BM (total concentration of 12 mg mLꢀ1) was spin-coated
on the top of prepared ITO/PEDOT:PSS substrate at 1500 rpm sꢀ1
for 60 s. Finally, an aluminium layer was deposited by thermal
evaporation under vacuum (ca. 10ꢀ4 Pa) through a mask, yielding
eight individual devices with 5 mm2 effective area. The devices for
hole mobility measurement were fabricated with an architecture of
ITO/PEDOT:PSS/SMs-donor:PC61BM/Au.
2.4. Synthesis and characterizations
The synthetic routes of the investigated compounds are shown
in Scheme 2.
2.3. Measurements and characterization
Compound A was prepared according to the procedure reported
in the literature [9], and the synthetic details of compounds D1, D2,
1H NMR and 13C NMR spectra were measured with a Bruker
Scheme 1. The molecular design strategy and chemical structures of TPADOBT, CZDOBT, TPAEDOBT and CZEDOBT.

L. Wang et al. / Dyes and Pigments 140 (2017) 203e211
205
Scheme 2. Synthetic routes of TPADOBT, CZDOBT, TPAEDOBT and CZEDOBT.
D3 and D4 are given in ESI [17]. The synthesis of TPADOBT, CZDOBT,
TPAEDOBT and CZEDOBT are described in the following sections:
4.23 (t, J ¼ 7.0 Hz, 4H), 1.90e2.03 (m, 8H), 1.27e1.58 (m, 32H),
0.86e0.90 (m, 12H). 13C NMR (100 MHz, CDCl3, ppm):
151.65,
d
151.08, 140.98, 140.20, 131.98, 125.98, 125.67, 124.21, 123.40, 122.91,
121.99, 120.55, 119.09, 117.83, 117.49, 109.01, 108.92, 74.51, 43.27,
31.90, 31.60, 30.58, 29.69, 29.38, 29.01, 26.99, 26.19, 22.69, 22.56,
14.09, 14.01. HRMS (MALDI-TOF): 1054.5344, [Mþ] (calcd for
2.4.1. Synthesis of compound TPADOBT
A mixture of compound D1 (184 mg, 0.5 mmol), compound A
(178 mg, 0.25 mmol), Na2CO3 (848 mg, 8.0 mmol) and Pd(PPh3)4
(4 mg, 0.013 mmol) in a solution of toluene (8 mL), H2O (4 mL) and
EtOH (2 mL) was refluxed at 110 ꢁC for 24 h. After being cooled to
room temperature, the reaction solution was poured into 50 mL
water, extracted with dichloromethane (DCM, 3 ꢂ 20 mL). The
combined organic layer was dried over anhydrous sodium sulfate.
The solvent was evaporated under reduced pressure and the res-
idue was purified by column chromatography eluting with petro-
leum ether (PE): DCM (2:1, v/v) to give a purple solid (241 mg,
C66H78N4O2S3: 1054.5287).
2.4.3. Synthesis of compound TPAEDOBT
A mixture of compound A (143 mg, 0.20 mmol), CuI (2 mg,
0.01 mmol), Pd(PPh3)2Cl2 (4 mg, 0.005 mmol) and compound D2
(135 mg, 0.5 mmol) in 20 mL THF and 15 mL Et3N was refluxed at
70 ꢁC for 12 h. The solvent was evaporated and the residue was
purified by column chromatography on silica gel column eluting
with PE: DCM (4: 1, v/v) to give a red solid (164 mg, yield: 75%),
yield: 92%), M.p.: 78e80 ꢁC. 1H NMR (400 MHz, CDCl3, ppm):
d 8.50
(d, J ¼ 1.2 Hz, 2H), 7.58 (d, J ¼ 8.4 Hz, 4H), 7.34 (d, J ¼ 1.6 Hz, 2H),
7.28 (t, J ¼ 7.8 Hz, 8H), 7.14 (d, J ¼ 7.6 Hz, 8H), 7.09 (d, J ¼ 8.4 Hz, 4H),
7.05 (t, J ¼ 7.2 Hz, 4H), 4.16 (t, J ¼ 7.0 Hz, 4H), 1.95e1.97 (m, 4H),
1.27e1.52 (m, 20H), 0.86 (t, J ¼ 6.8 Hz, 6H). 13C NMR (100 MHz,
M.p.: 60e62 ꢁC. 1H NMR (400 MHz, CDCl3, ppm):
d 8.52 (d,
J ¼ 4.0 Hz, 2H), 7.39 (d, J ¼ 8.8 Hz, 4H), 7.34 (d, J ¼ 4.0 Hz, 2H), 7.29
(t, J ¼ 7.8 Hz, 8H), 7.13 (d, J ¼ 7.6 Hz, 8H), 7.08 (t, J ¼ 7.4 Hz, 4H), 7.02
(d, J ¼ 8.8 Hz, 4H), 4.14 (t, J ¼ 7.0 Hz, 4H), 1.99e1.95 (m, 4H),
CDCl3, ppm):
d151.61, 150.96, 147.49, 147.42, 145.50, 133.04, 131.92,
1.29e1.49 (m, 20H), 0.88 (t,
(100 MHz,CDCl3, ppm): d 151.93, 150.73, 148.11, 147.15, 135.28,
J
¼
6.6 Hz, 6H). 13C NMR
129.34, 128.50, 126.63, 124.62, 123.63, 123.18, 122.24, 117.43, 74.46,
31.85, 30.48, 29.60, 29.36, 26.14, 22.69, 14.12. HRMS (MALDI-TOF):
1042.4313, [Mþ] (calcd for C66H66N4O2S3: 1042.4348).
132.39, 131.36, 130.89, 129.43, 125.35, 125.09, 123.66, 122.16, 117.34,
115.74, 95.33, 82.42, 74.59, 31.83, 30.32, 29.50, 29.31, 25.97, 22.68,
14.11. HRMS (MALDI-TOF): 1090.4269 [Mþ] (calcd for
C70H66N4O2S3: 1090.4348).
2.4.2. Synthesis of compound CZDOBT
A mixture of compound D3 (189 mg, 0.5 mmol), compound A
(178 mg, 0.25 mmol), Na2CO3 (848 mg, 8.0 mmol) and Pd(PPh3)4
(4 mg, 0.013 mmol) in a solution of toluene (8 mL), 4 mL H2O and
2 mL EtOH was refluxed at 110 ꢁC for 24 h. After being cooled to
room temperature, The reaction solution was poured into 50 mL
water, extracted with DCM (3 ꢂ 20 mL) and the combined organic
layer was dried over anhydrous sodium sulfate. The solvent was
evaporated and the residue was purified by column chromatog-
raphy on silica gel column eluting with PE: EA (2: 1, v/v) to give a
red solid (211 mg, yield: 80%), M.p.: 86e89 ꢁC. 1H NMR (400 MHz,
2.4.4. Synthesis of compound CZEDOBT
A mixture of compound A (143 mg, 0.20 mmol), CuI (2 mg,
0.01 mmol), Pd(PPh3)2Cl2 (4 mg, 0.005 mmol), D4 (137 mg,
0.5 mmol) in 20 mL THF and 15 mL Et3N was refluxed at 70 ꢁC for
24 h. The solvent was evaporated and the residue was purified by
column chromatography on silica gel column eluting with PE: DCM
(4: 1, v/v) to give a purple solid (132 mg, yield: 60%), M.p.:
75e80 ꢁC. 1H NMR (400 MHz, CDCl3, ppm):
d
8.56 (d, J ¼ 3.6 Hz, 2H),
8.32 (s, 2H), 8.11 (d, J ¼ 7.6 Hz, 2H), 7.65 (d, J ¼ 8.4 Hz, 2H), 7.49 (t,
J ¼ 7.6 Hz, 2H), 7.37e7.42 (m, 6H), 7.27 (d, J ¼ 7.2 Hz, 2H), 4.29 (t,
J ¼ 7.2 Hz, 4H), 4.19 (t, J ¼ 7.0 Hz, 4H), 1.85e2.03 (m 8H), 1.26e1.55
CDCl3, ppm):
d
8.45 (m, 4H), 8.17 (d, J ¼ 7.6 Hz, 2H), 7.85 (s, 2H), 7.49
(t, J ¼ 7.6 Hz, 4H), 7.43 (t, J ¼ 6.8 Hz, 4H), 7.28 (s, 2H), 4.32 (s, 4H),

206
L. Wang et al. / Dyes and Pigments 140 (2017) 203e211
(m, 32H), 0.85e0.89 (m, 12H). 13C NMR (100 MHz, CDCl3, ppm):
151.93, 150.79, 140.85, 140.24, 135.07, 131.21, 130.91, 129.14, 126.16,
Accordingly, the exceptional thermal stability of these materials
could provide a guarantee for their PV applications [18].
d
125.73, 123.95, 122.92, 122.49, 120.56, 119.41, 117.37, 112.96, 108.97,
108.80, 96.55, 81.37, 74.60, 43.26, 31.86, 31.56, 30.35, 29.54: 29.35,
28.95, 26.96, 26.00, 22.71, 22.55, 14.13, 14.00. HRMS (MALDI-TOF):
1102.5364 [Mþ] (calcd for C70H78N4O2S3: 1102.5287).
3.2. Optical properties
The solar absorption ability is preliminarily investigated with
the UVeVis absorption spectra of these compounds in chloroform
solution and in thin film. As shown in Fig. 1, all compounds show
two primary absorption bands at low and high energy region,
covering 300e600 nm in solution and 300e700 nm in film
respectively.
3. Results and discussion
3.1. Synthesis and thermal stability properties of materials
The synthetic routes of TPADOBT, CZDOBT, TPAEDOBT and
CZEDOBT are outlined in Scheme 2. Starting from TPA and Cz, D1
and D3 were attained by Miyaura borylation reaction via bis(pi-
nacolato)diborane with monobromo-substituted TPA and Cz,
respectively, using potassium acetate as base and Pd(PPh3)2Cl2 as
catalyst in toluene. D2 and D4 were synthesized by Sonogashira
In solution, the quantitative UVeVis absorption spectra of them
were obtained in chloroform solution at a concentration of
1 ꢂ 10ꢀ5 mol Lꢀ1, as shown in Fig. 1(a). These compounds all pre-
sent similar absorption spectra due to their similar D-
p-A-
p-D
sol
typed molecular backbone. The maxima absorption peaks (
l
) at
max
low energy region of TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT
were observed at 511, 503, 522 and 520 nm, respectively, resulting
from intermolecular charge transfer (ICT) transition between mo-
lecular terminal donor and central acceptor unit [19]. The pertinent
molar extinction coefficient (ε) of them are listed in Table 1. As a
consequence, the higher ε and stronger absorption were obtained
for TPAEDOBT and CZEDOBT in comparison with another two
single-bond compounds. The results indicate that the compounds
with ethynyl-linker possess relatively stronger absorption capacity
than single-bond compounds, probably contributing to the
improvement of short-circuit current density (JSC) [20]. By contrast,
the absorption bands of these compounds in their thin film state
are broadened and the absorption peaks (lfimlmax) are red-shifted by
approximately 20 nm, suggesting the enhanced intermolecular
cross-coupling
reaction
via
ethynyltrimethylsilane
with
monoiodide-substituted TPA and Cz, respectively, in the presence
of Et3N, Pd(PPh3)2Cl2 and CuI in THF, and more detailed synthetic
procedure and characterization are described in Scheme S1 of ESI.
The central DOBT-based acceptor unit of compound A was prepared
according to previously reported procedure. Finally, TPADOBT and
CZDOBT were synthesized by Pd-assisted Suzuki coupling reaction
in high yields of 92% and 80%, respectively. TPAEDOBT and CZE-
DOBT were synthesized by Sonogashira coupling reaction in yields
of 75% and 60%, respectively. Their chemical structures were
determined with 1H NMR, 13C NMR and HRMS, which were
consistent with their proposed structures. Four materials possess
good solubility in common organic solvents, which meets the
requirement for fabrication of solution-processed OPV devices. The
thermal stability properties were studied by thermogravimetric
analysis (TGA), as shown in Fig. S19 of ESI. The onset decomposition
temperatures of TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT with
5% weight-loss are 329 ꢁC, 328 ꢁC, 324 ꢁC and 314 ꢁC, respectively.
interaction for solid-state packing [21]. The optical band gaps (Eogpt
)
estimated from the absorption onsets of TPAEDOBT, CZEDOBT,
TPADOBT and CZDOBT in film state are 2.04, 2.08, 2.00 and 2.05 eV
respectively, which are in a desirable range for OPV applications
[22].
Fig. 1. (a) The quantitative UVeVis absorption spectra of TPADOBT, CZDOBT, TPAEDOBT and CZEDOBT in chloroform solution at a concentration of 1 ꢂ 10ꢀ5 mol Lꢀ1; (b) UVeVis
spectra of these compounds in thin films normalized in the region of long wavelength. .
Table 1
The photophysical, electrochemical and theoretically calculated data of compounds TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT.
(nm)/ε (Mꢀ1 cmꢀ1
)
l
(nm) Eogpt (eV) Ksv (Mꢀ1
)
HOMOCV (eV) LUMOCV (eV) EgCV (eV) HOMODFT (eV) LUMODFT (eV) EgDFT (eV)
sol
film
l
max
max
TPAEDOBT 367/9.08, 495/5.81
511
503
522
520
2.04
2.08
2.00
2.05
3103
3048
2984
2840
ꢀ5.29
ꢀ5.31
ꢀ5.12
ꢀ5.14
ꢀ3.24
ꢀ3.21
ꢀ3.15
ꢀ3.11
2.05
2.10
1.97
2.03
ꢀ4.66
ꢀ4.67
ꢀ4.59
ꢀ4.63
ꢀ2.50
ꢀ2.39
ꢀ2.35
ꢀ2.25
2.16
2.28
2.24
2.38
CZEDOBT
TPADOBT
CZDOBT
352/8.91, 493/5.67
372/6.55, 507/4.12
351/4.74, 499/3.52

L. Wang et al. / Dyes and Pigments 140 (2017) 203e211
207
To evaluate the photoinduced charge separation process be-
tween donor and acceptor, the fluorescence quenching experiment
was carried out with the synthesized donor materials and PC61BM.
As expected, the fluorescence quenching phenomenon of TPAE-
DOBT, CZEDOBT, TPADOBT and CZDOBT was clearly revealed. The
fluorescence intensity of them gradually decreased with the
increasing concentration of PC61BM, as shown in Fig. 2, indicating
an efficient charge separation process between the synthesized
donors and acceptor PC61BM. Moreover, the dependence of fluo-
rescence intensity on the concentration of quencher PC61BM is
linear at low concentrations in conformity with the Stern-Volmer
equation [23]: F0/F ¼ 1 þ KSV [C], where F0 and F represent the
emission intensity in the absence and presence of PC61BM,
respectively, KSV is the fluorescence quenching constant, and [C] is
the concentration of PC61BM. According to Stern-Volmer equation,
the fitting plots of these compounds are shown in inset of Fig. 2.
Accordingly, the KSV values of TPAEDOBT, CZEDOBT, TPADOBT and
CZDOBT are 3103, 3048, 2984 and 2840 Mꢀ1, respectively. All ma-
terials present high quenching constant, the efficient charge sep-
aration process is further approved. By contrast, the charge
separation of TPAEDOBT is more efficient relative to other three
compounds due to its relatively higher Ksv value, probably
contributing to its enhanced PV performance.
3.3. Electrochemical characteristics
Cyclic voltammetry (CV) experiment was performed to inves-
tigate the electrochemical property and energy levels of these
compounds (measurement details are recorded in experimental
section). The cyclic voltammogram of them in 0.1 M Bu4NBF4/
CH2Cl2 solution are shown in Fig. 3, and the corresponding elec-
trochemical data are summarized in Table 1. The onset oxidation
potential (Eox) at 0.37 and 0.39 V is obtained in the anode potential
region for single-bond compounds TPADOBT and CZDOBT, corre-
sponding to the oxidation of TPA and Cz unit, respectively. By
contrast, the triple-bond compounds TPAEDOBT and CZEDOBT
exhibit the greatly increased Eox at 0.54 and 0.56 V, respectively,
attributing to the electron-withdrawing character of ethynyl-linker
[24]. In addition, a slightly increased Eox is observed for compounds
CZDOBT and CZEDOBT with weakened electron-donating unit (Cz)
relative to TPADOBT and TPAEDOBT. On the other hand, the onset
reduction potentials (Ered) at ꢀ1.60 and ꢀ1.64 V is observed in the
cathodic potential region for TPADOBT and CZDOBT respectively,
corresponding to the reduction of DOBT. In contrast to two single-
bond compounds, TPAEDOBT and CZEDOBT exhibit the greatly
increased Ered at ꢀ1.51 and ꢀ1.54 V respectively, also attributing to
the electron-withdrawing character of ethynyl-linker. In contrast to
the reduction behaviour of TPADOBT and TPAEDOBT, a slightly
increased Ered is observed for compounds CZDOBT and CZEDOBT
owing to the weakened electron-donating unit (Cz). As a result, the
increasing of oxidation and reduction potential values of TPAE-
DOBT and CZEDOBT relative to TPADOBT and CZDOBT can be
rationally attributed to the molecular design with ethynyl-linker.
The electrochemical band gaps (ECgV) and the corresponding en-
ergy levels (HOMOCV and LUMOCV) could be achieved from Eox and
Ered (calculation details are described in experimental section).
Consequently, the HOMOCV levels for TPAEDOBT, CZEDOBT, TPA-
DOBT and CZDOBT are ꢀ5.29, ꢀ5.31, ꢀ5.12 and ꢀ5.14 eV, and their
LUMOCV levels are ꢀ3.24, ꢀ3.21, ꢀ3.15 and ꢀ3.11 eV, respectively.
These data indicate that ethynyl-linker is very effective in lowering
both the HOMO and LUMO levels of the materials without signifi-
cantly costing their narrow band gaps [25]. The electrochemical
energy-level diagram of these compounds together with PC61BM
was schematically shown in Fig. S20 of ESI. The results clearly
demonstrate that all these compounds in principle can behave as
donor materials with PC61BM as acceptor in solution-processed
OPV cells [26]. By contrast, TPAEDOBT and CZEDOBT possess
Fig. 2. Fluorescence emission spectra of donor materials TPAEDOBT, CZEDOBT, TPA-
DOBT and CZDOBT in chloroform solution (1.0 ꢂ 10ꢀ5 M) with increasing concentra-
tion of acceptor PC61BM (ꢂ10ꢀ5 M): 0.0 (0), 3.0 (1), 5.0 (2), 8.0 (3), 10.0 (4), 15.0 (5),
20.0 (6), 25.0 (7); The insets are fitting plots for four compounds respectively by Stern-
Volmer equation.
Fig. 3. Cyclic voltammogram of TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT in 0.1 M Bu4NBF4/CH2Cl2 solution under nitrogen atmosphere with a scan rate of 100 mV sꢀ1
.

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L. Wang et al. / Dyes and Pigments 140 (2017) 203e211
obviously lower HOMOCV levels, which could probably ensure the
high VOC. Among them, CZEDOBT exhibits a lowest HOMOCV level
in contrast to the other three compounds, which should be
attributed to the synergistic effect of ethynyl-linker and relatively
weakened electron-donating ability of Cz units. Herein, remarkably
improvement of VOC values in CZEDOBT based device will be ex-
pected. Moreover, their ECgV are estimated to be about 2.05, 2.10,1.97
and 2.03 eV, respectively, which are in good agreement with their
optical band gaps.
probably play an important role in stabilizing of separated hole from
excitons and thus improve the hole transporting capacity [29].
The calculated HOMODFT for TPAEDOBT, CZEDOBT, TPADOBT and
CZDOBT are ꢀ4.66, ꢀ4.67, ꢀ4.59 and ꢀ4.63 eV, and the corre-
sponding LUMODFT are ꢀ2.50, ꢀ2.39, ꢀ2.35 and ꢀ2.25 eV, respec-
tively. Thus the HOMODFT values for TPAEDOBT and CZEDOBT
decreased 0.07 and 0.04 eV, while the LUMODFT values also reduced
0.15 and 0.14 eV in contrast to that of TPADOBT and CZDOBT. These
results imply that HOMO and LUMO energy levels can be reduced
simultaneously without sacrificing the Eg via incorporating ethynyl-
linker on molecular backbone, which could potentially contribute to
the VOC and JSC [30]. Despite the discrepancies exist between
computational analysis and experimental results, theoretical cal-
culations for a series of closely related materials have proven to be
crucial to understanding the strategy of molecular design [31].
The calculated optical absorption profiles and electronic tran-
sitions of TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT are shown
in Fig. 4(b), and the calculated data of electronic transitions using
TD-DFT method are summarized in Table S1 of ESI. The predicted
absorption spectra of these compounds show two primary bands at
short and long wavelength, which are consistent with their UVeVis
absorption spectra. The long wavelength absorption band corre-
lates with the electron transition between the HOMO and LUMO,
while the short one is primarily a result of HOMO to LUMOþ1
transition. Despite with some margin of error in TD-DFT predicted
profiles, the trend could be reliably computed for these closely
related compounds [32]. These results could preliminarily guide
the strategy of molecular design.
3.4. Theoretical calculations
In order to further demonstrate the strategy of molecular design,
the density functional theory (DFT) and the time-dependent density
functional theory (TD-DFT) calculations were performed on these
compounds to analyze their molecular orbital depictions, energy
levels, electronic transitions and optical absorption profiles using
Gaussian 09 software at the Becke's three-parameter gradient-cor-
rected functional (B3LYP) with a polarized 6-31G(d) basis [27]. A
summary of the computational analysis for TPAEDOBT, CZEDOBT,
TPADOBT and CZDOBT using DFT/TD-DFT methods is illustrated in
Fig. 4, and the calculated data of energy levels (HOMODFT and
LUMODFT) and band gaps (EDg FT) are also summarized in Table 1.
Fig. 4(a). clearly presents the schematic diagram of the calculated
energy levels, frontier molecular orbital and electronic-density
distributions of these molecules. The electronic densities of HOMO
are similarly distributed the entire molecular backbone without a
significant biased distribution, whereas those of LUMO are pre-
dominantly located on the central DOBT, revealing an apparent ICT
process between molecular terminal donor and central acceptor
unit [28]. Moreover, the electronic densities of HOMO-1 are located
on their terminal electron-donating groups with a greater propor-
tion. This feature reveals that the terminal donor unit could
3.5. Hole mobility
Hole-mobility (mh) is often considered as one of the most
important physical parameters for donor material design. The mh of
Fig. 4. Summary of the computational analysis for TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT using DFT/TD-DFT methods. (a) calculated energy levels, molecular orbital de-
pictions and electronic-density distributions; (b) predicted optical absorption profiles and calculated electronic transitions.

L. Wang et al. / Dyes and Pigments 140 (2017) 203e211
209
Table 2
each material was achieved in the dark with a structure of ITO/
PEDOT:PSS/SMs-donor:PC61BM/Au, as shown in Fig. 5. As you can
see, the J-V curves of hole-only devices were recorded in Fig. 5(a)
and the mh values can be determined in a double logarithmic scale
by fitting the J-V curves to a SCLC model using the Mott-Gurney law
(the method is described in experimental section). Accordingly, the
fitting plots of the data points in ohmic regime and in SCLC regime
are depicted in Fig. 5(b), and the mh values can be obtained in the
SCLC region with a slope of 2 [33]. As listed in Table 2, the derived mh
values of TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT are
The hole mobility data and PV parameters of the devices based on TPAEDOBT,
CZEDOBT, TPADOBT and CZDOBT.
mh (cm2 Vꢀ1 sꢀ1
)
VOC (V)
JSC (mA cmꢀ2
)
FF
PCE (%)
TPAEDOBT
CZEDOBT
TPADOBT
CZDOBT
1.32 ꢂ 10ꢀ3
6.00 ꢂ 10ꢀ4
3.60 ꢂ 10ꢀ4
5.65 ꢂ 10ꢀ5
1.01
1.06
0.92
0.95
8.35
6.68
5.93
5.66
0.38
0.30
0.34
0.34
3.21
2.10
1.85
1.82
1.32 ꢂ 10ꢀ3, 6.00 ꢂ 10ꢀ4, 3.60 ꢂ 10ꢀ4 and 5.65 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1
,
relatively higher JSC values of triple-bond compounds could be
rationally attributed to their relatively higher molar extinction co-
efficients and more efficient carrier transporting capacity. This
result is consistent with their UVeVis absorption and their hole
mobility. Meanwhile, the relatively higher Jsc values of TPA-base
compounds relative to Cz-based compounds can be also assigned
to the relatively narrower Eg, higher ε and mh values. In contrast to
0.92 V of TPADOBT and 0.95 V of CZDOBT, the increased VOC values
of 1.01 and 1.06 V are achieved for the devices based on TPAEDOBT
and CZEDOBT, respectively. The higher VOC values result from their
relatively higher electrochemical Eox and deeper-lying HOMO levels
induced by rational molecular design.
It is worth mentioning that an impressive VOC of 1.06 V has been
observed for CZEDOBT-based device most likely due to synergistic
effect of ethynyl-bridge with the electron-withdrawing character
and Cz units with the relatively weakened electron-donating abil-
ity. Moreover, the PCEs of 1.85% and 1.82% were obtained for single-
bond compounds TPADOBT and CZDOBT. While the devices based
on triple-bond compounds TPAEDOBT and CZEDOBT exhibited a
superior PCE of 3.21% and 2.10% respectively. Remarkably among
them, the best PV performance was obtained for TPAEDOBT-based
device, most likely arising from its triple-bond effect and its
remarkably high hole transporting capacity, since a consistent
trend was observed for their PCEs and mh values. Bearing in mind
that the morphology of the active layer also has important effect on
the performance of device. AFM measurement was carried out to
investigate the morphology properties and the interesting results
were revealed. TPAEDOBT exhibited relatively higher Jsc, FF and
PCE values, which were consistent with its better bicontinuous
interpenetrating networks as shown in AFM image. On the con-
trary, CZDOBT with the worse morphology exhibited relatively
lower PCE value (ESI, Fig. S22).
respectively.
As literature reported, the hole-transporting capacity of PV-
donor materials might be closely related to their unique chemical
structure and the intermolecular interaction of solid film [34]. As
expected, the mh values of TPAEDOBT and CZEDOBT are much
greater than those of TPADOBT and CZDOBT, indicating that the
triple-bond compounds possess much favoured molecular struc-
ture and enhanced intermolecular interaction induced by improved
coplanarity and rigidity with ethynyl-linker. By contrast, TPAE-
DOBT-based device possesses the optimal hole transporting ca-
pacity in accordance with its greatest mh value among them.
Supplementary evidence could be provided by its relatively higher
degree of crystallinity from X-ray diffraction analysis (XRD patterns
are shown in Fig. S21 of ESI). The results would probably provide
some valuable hints on their PV performance, especially the JSC and
PCEs.
3.6. Photovoltaic properties
To fully evaluate the PV property of new materials and reveal the
relations between the property and molecular structure, a series of
BHJ devices with a typical architecture of ITO/PEDOT:PSS/SMs-
donor:PC61BM/Al were fabricated in chloroform solution. We first
performed the optimization of the devices based on SMs-
donor:PC61BM blends with varied D/A weight ratios (1:1, 1:2
and1:3). Accordingly, the data of PV performance are summarized
in Table S2 of the ESI documents. Remarkably, the device with the
D/A weight ratio of 1:2 showed the best PV performance for each
material. The related current density voltage (J-V) characteristics of
the PV devices with the D/A weight ratio of 1:2 are sketched in
Fig. 6, and the pertinent device parameters are listed in Table 2. As a
result, the JSC of TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT are
8.35, 6.68, 5.93 and 5.66 mA cmꢀ2, respectively. By contrast, the
In order to further verify PV performance, the IPCE spectra of
related devices based on TPAEDOBT, CZEDOBT, TPADOBT and
CZDOBT were tested as well. As shown in Fig. 6(b), all spectra
Fig. 5. Hole mobility of the compounds TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT. (a) J-V curves of the hole-only devices tested in the dark with an architecture of ITO/
PEDOT:PSS/SMs-donor:PC61BM (1:2, w:w)/Au; (b) J-V curves of the devices in a double logarithmic scale, and the solid lines are fitting plots of the data points in ohmic regime and
in SCLC regime.

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L. Wang et al. / Dyes and Pigments 140 (2017) 203e211
Fig. 6. PV properties of the investigated compounds TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT. (a) J-V characteristics of the PV devices with a structure of ITO/PEDOT: PSS/SMs-
donor:PC61BM (1: 2, w: w)/Al under an illumination of AM 1.5G (100 mW cmꢀ2). Inset: schematic structure of PV device; (b) The IPCE spectra of the devices.
exhibit a first peak around 350 nm followed by a second broad band
with maximum value at around 490 nm, which are in agreement
with their absorption profiles. A first maximum of ca. 72%, 60%, 50%
and 43% in the region of 300e400 nm and a second maximum of ca.
72%, 59%, 42% and 44% in the region of 400e700 nm were obtained
for TPAEDOBT, CZEDOBT, TPADOBT and CZDOBT, respectively. As a
result, the IPCE values show consistent trend with their JSC values.
Among them, the higher IPCE values were obtained for devices
based on TPAEDOBT and CZEDOBT compared to those of two
reference compounds. In particular, it is worth pointing out that
TPAEDOBT based device showed a highest IPCE value, which is
responsible for its best JSC and PCE in comparison. All integrated JSC
values from the IPCE spectra show within 5% mismatch compared
with the JSC from the J-V curves. Further experimental work on
device optimization are underway in our team.
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a diketopyrrolopyrrole core for molecular bulk